Ca efflux from squid giant axons will be studied by dialyzing a solution of EGTA/CaEGTA into the axoplasm with the composition of the fluid adjusted to provide a free (Ca) of from 10 to the seventh power to 10 to the fourth power M. At selected concentrations in the range indicated above, the (Na)i will be varied over the range 0-100mM. Ca and Na are known to compete for passage at the outside of the membrane. This study will characterize their competition at the inside surface of the cell.